Turning to FIG. 1, an example of a conventional two-stage Doherty-type power amplifier (PA) 100 can be seen. PA 100 generally comprises a carrier amplifier 102, a peak amplifier 104, and a combiner 106 (which can be a node). In operation, the carrier amplifier 102 provides amplification to an input signal at low amplitude, and, as the carrier amplifier begins to saturate, the peak amplifier 104 provides additional amplification to allow for a greater overall amplitude range. A reason for having this configuration is that the efficiency for a Doherty amplifier (i.e., PA 100) is greater than a traditional amplifier (i.e., class B amplifier). An example of the difference in efficiency between PA 100 (having two and three stages), a class A amplifier, a class AB amplifier, and a class B amplifier can be seen in FIG. 2. Additionally, multiple amplifiers or amplifier sets (i.e., N-way Doherty amplifiers) can be used as PA 100.
There are some drawbacks to the Doherty configuration. For many applications, such as telecommunications, baseband signals are predistorted to compensate for nonlinearities in the power amplifiers (i.e., PA 100), but because there is a “switching event” in a Doherty-type amplifier when the peak amplifier (i.e., peak amplifier 104) becomes active, the nonlinearities become dynamically varying. As a result, DPD becomes increasingly difficult to perform. Therefore, there is a need for DPD correction of for a Doherty-type PA or other amplifiers that are sensitive to output power levels (i.e., dynamically varying PAs).
Some examples of conventional solutions are: U.S. Pat. No. 7,366,252; U.S. Patent Pre-Grant Publ. No. 2007/0164818; and U.S. Patent Pre-Grant Publ. No. 2008/0111622.